Arguably the most difficult part of building a stone arch bridge is “turning the arch.” This phrase simply refers to keeping the correct angle on each individual stone so that the arch goes in a circle — not as easy, perhaps, as one might think.
The Gothic Arch
To begin with, some arch shapes are easier to turn than others. The Gothic arches in medieval bridges are surprisingly forgiving. As they terminate in a point at the top, this point takes much of the necessary angle to be made, allowing the sides of the arch (called the “haunches”) to be fairly straight.
The Flat Arch
Flat arches are also easier to turn than highly rounded ones for the same reason.
Any given arch span has a total angle of 180 degrees. With a Roman arch, the whole arch is a 180-degree segment of a circle; in the case of a flatter arch, such as, say, a 90-degree segment of a circle, the arch rises from two 45-degree angles on each abutment, thereby achieving 180 degrees total. Therefore, with a flat arch, most of the turning is done on triangular stones on the abutments, rather than the arch proper.
Using Thin Stones
A rather ingeniously simple method of turning an arch was prominent in medieval times. To begin with, each individual stone must have the correct angles cut on its face to achieve the correct curvature. Also, the bottom of the stone is ideally rounded so that it will rest on the round formwork properly. With large stones, these angles must, of necessity, be quite pronounced.
So, for a Roman arch with, say, 10 stones, each stone must achieve a slope of 18 degrees (18 * 10 = 180). But what if, say, 20 stones were used? Now each stone only needs to achieve an angle of 9 degrees — closer to a straight line.
Hence, the expedient of using very thin stones comes into play. The thinner the stones, the closer to flat they may be, hence the easier these stones become to produce. It is even possible, with judicious picking of stones, to build the arch with no shaping of the stones required.
The Problem With Arches
Now, it is still improbable that all the stones, unless they are cut, fit perfectly. A simple expedient used is to wedge small pieces of stones in the gaps between the bigger ones.
While not as ideal as a perfectly cut stone, this method is quite successful.
Frequently, to keep the chips from working out, mortar is used — not as glue but rather to ensure a perfect fit. In medieval times the mortar was frequently poured in after the arch was finished.
Another similar method is to use loose-fitting, roughly cut stones, with stone chips between them to create the angle.
A compromise solution used by Walter Sharp is to cut the stones on the face of the arch to a precise fit and leave the stones in between a loose fit, with grout used to hold the angle. The main trouble with this was that the grout used deteriorated over time.
The advantage of leaving the faces of the arch perfect when the rest is imperfect is that the faces are susceptible to damage from debris borne in the water, and hence the “joints” are less vulnerable than they would be if composed of mortar-held stone chips.
This method of using stone chips can be taken to an extreme, however. It is best if the stones at least fit reasonably close, or the arch can quickly become very weak.